The directional control valve capable of monitoring the changeover operation of a spool utilizing a magnet is well known as disclosed in, for example, Japanese Examined Utility Model Publication No. 7-31021(Japanese Unexamined Utility Model Publication No. 2-88079). This directional control valve is provided on both ends of a spool with respective pistons for receiving pilot fluid pressure, and is adapted to change over the spool by the fluid pressure acting on the pistons. This directional control valve has a magnet mounted on one piston, and has a detection coil for detecting the change in magnetic flux, installed at a position opposite to the magnet mounted on a casing, whereby the directional control valve detects the moving speed of the piston, or the spool from the magnitude of the induced voltage generated in the detection coil by the change in magnetic flux when the magnet moves together with the piston, and judges whether the moving speed is normal or not.
However, since the above-described conventional directional control valve is constituted so that the magnet is installed at a position which is exposed to the pressure chamber adjacent to an end face of the piston, the magnet will directly contact a pilot fluid. Therefore, when the fluid contains water, chemical mist, particulates of magnetic material such as metallic powder, or the like, there has often arisen the problem that the contact of the magnet with these substances makes the magnet rust, corrode, or adsorb the particulates. This would bring about drawbacks of reducing the detecting accuracy due to the decrease in magnetic force, or incurring poor sliding conditions.
Furthermore, the above-described valve is constituted so as to make the detection coil generate an induced voltage in response to the change in magnetic flux with the movement of the magnet, and to detect the moving speed of the spool from the magnitude of the induced voltage to judge whether the moving speed is normal or not, but can not detect operating positions of the spool.